The present invention relates generally to an automatic writing instrument which is constructed to feed the writing lead into a writing position merely through application of pressure or thrust against the writing point end of the pencil, or against the writing tip of lead itself. More particularly, the present invention relates to a mechanical pencil of the type described above in which, when a lead portion exposed from the pencil point end is used up, lead is fed into a writing position by merely releasing a small pressure applied against the writing point end without changing the writing posture of the pencil so that the lead is fed to the writing position continuously during a writing operation. The present invention is applicable to an automatic drawing machine.
Many attempts have been made to cause the lead to feed into a writing position without involvement of a special pushing actuation of a button which is generally provided at the rear end of the pencil. Some of these attempts are disclosed in U.S. Pat. No. 4,504,163 issued Mar. 12, 1985 to the present assignee, and such mechanical pencils as described above are generally classified into two types of pencils from the viewpoint of mechanism and operation.
The first type of mechanical pencil is disclosed in U.S. Pat. No. 4,504,163 in which a slider and a chuck mechanism are retracted within the pencil through the lead to a predetermined distance and then these elements are advanced or returned to their original forward position by releasing a pressure which has been applied to the writing point end. At this moment, the slider is held or temporarily suspended at its retracted position and, on the other hand, the chuck mechanism is immediately caused to advance, and thereafter the slider is caused to advance to its original position. By the delayed movement of the slider relative to the chuck mechanism, the lead is fed to the writing position of the pencil.
The second type of pencil is disclosed in Japanese Patent Application No. 59-25906, filed Feb. 14, 1984 by the present inventor, in which the distance of rearward and then forward displacement of the slider is set so as to be greater than that of a chuck mechanism so that the lead can be pulled forwardly out of the chuck when the slider is returned forwardly.
Both the first and second types of the pencil have advantages and provide improvement of the lead-feed operation. Although these pencils produce a reliable lead-feed operation while the pencil is continuously held in a writing posture without a special pushing operation, on the other hand, a relatively large pressure which is greater than a normal writing pressure must be applied and, therefore, continuous writing is interrupted and the relatively large pressure applied to the pencil point end sometimes injures the writing surface of a paper. In order to overcome such a disadvantage, an attempt has been made to use springs of smaller spring force so that a relatively small force substantially equal to a normal writing pressure can provide a lead feed operation, but this causes a new problem that the lead is projected as writing pressure is applied.